I have been using a quite secure setup for the last couples of years with a 4 drive RAID 6 setup. This setup can tolerate two disk failures without any data loss. Recently though, I have been getting close to the edge of the filesystem and could use some extra space and since I have both monthly backup to an external hard drive and nightly offsite backup I am actually not very afraid of a data loss on a RAID 5 setup. So I have planned to change my 4 disk RAID 6 to a 4 disk RAID 5 without any spares.
A word of caution: Please do not do any of the actions below before a backup has been made.